Size reduction in metal oxide semiconductor (MOS) transistors has resulted in improved speed, performance and density of transistors. However, in some instances, integrated circuits (ICs) include critical regions where even greater switching speed of MOS transistors is desirable for the ICs to perform as intended. A critical region, for example, is a circuit path having a larger number of comparisons, with respect to other non-critical regions, where the speed of the comparisons is increased so that an output of the critical region occurs at substantially the same time as an output from a different circuit having fewer comparisons. MOS transistor speed is substantially inversely proportional to a threshold voltage of the MOS transistor. That is as the threshold voltage decreases, switching speed for the MOS transistor increases.
One way to increase speed in the critical regions of ICs is to modify a pocket implantation region of the MOS transistors. Modification includes changing the size or dopant concentration of the pocket implantation region. Increased speed through modifying pocket implantation regions; however, increases current leakage exponentially with increased speed. Increased current leakage increases the power consumption of the IC. Also, additional implantation masks are needed for creating or modifying the pocket implantation regions. The use of additional masks increases production time and resources needed to produce the MOS transistors.
Another way to increase speed in the IC overall is to decrease the thickness of the gate dielectric of all the MOS transistors. Thinner gate dielectrics, however, also increase current leakage at speeds below about 1200 MHz. Thinner gate dielectrics are also susceptible to reliability issues such as negative bias temperature instability (NBTI) and hot carrier injection (HCI), which permanently alter the threshold voltage. Thinner gate dielectrics are also vulnerable to breakdown at high power levels, making thin gate dielectrics unsuitable for some MOS transistors, such as input/output (I/O) transistors.